Integral skin polyurethane foams are well known to those skilled in the art. Such foams typically have a cellular interior and a high density microcellular or noncellular skin. In general, integral skin foams are prepared from the reaction of an organic isocyanate with a substance having at least one isocyanate reactive group in the presence of one or more catalysts, blowing agents, and a variety of optional additives. The reaction is typically carried out in a mold where a higher density skin forms at the interface of the reaction mixture and the relatively cool interior surface of the mold.
Traditionally, integral skin polyurethane foams were expanded with the use of chlorofluorocarbons (CFCs) blowing agent or hydrogenated chlorofluorocarbons (HCFCs) blowing agents. However, because of environmental concerns, government regulations now mandate the reduction and eventual elimination of CFCs and HCFCs in integral skin foam production.
As a result, the polyurethane industry has devoted considerable time and energy in developing alternative blowing agents which produce integral skin foams having performance and processing properties that are at least equivalent to, or better than, those of traditional CFC and HCFC blowing agents.
It has been particularly difficult to find alternative blowing agents suitable for use in integral skin foams intended for use in shoe soles. Such foams must have a cosmetically acceptable appearance and a smooth surface with a good feel. In addition, they must exhibit enhanced resistance to abrasion and cracking on flex. Such particular properties must be in addition to optimum values of tensile strength, tear strength, and elongation.
One alternative blowing agent that has found particular favor with the makers of integral skin foams intended for use in shoe soles is 1,1,1,2-tetrafluoroethane or HFC-134a. Foams produced with HFC-134a alone or in combination with other blowing agents, such as water, exhibit generally adequate physical properties.
However, the use of HFC-134a has several problems. HFC-134a is a gas and is not readily soluble in typical polyurethane resin systems. As a result, special equipment is needed for addition. Processing problems become more acute as ambient temperatures increase. Also, environmental concerns about the global warming potential of HFC-134a have been raised.
Accordingly, manufacturers of integral skin polyurethane shoe soles are increasingly requesting alternatives to HFC-134a-blown polyurethane compositions. In particular, there is a great desire for a solely water blown polyurethane system capable of producing integral skin foams exhibiting performance properties suitable for use as shoe soles.
There have been several attempts to produce such solely water blown integral skin polyurethane compositions.
For example, WO 91/17197 discloses microcellular polyurethane polymers prepared from isocyanate-terminated poly(oxytetramethylene) glycol prepolymers. The isocyanate compound is a isocyanate-terminated prepolymer, having an isocyanate content of from 14 to 28 percent, which is obtained by reacting an isocyanate containing at least 70 weight percent 4,4'-methylene diphenyl isocyanate with a poly(oxytetramethylene) glycol which has an average hydroxyl equivalent weight of from 250 to 1500.
However, the WO 91/17197 specification teaches that the blowing agent need not comprise solely water and may contain halogenated hydrocarbons and preferably chlorofluorocarbons such as trichlorodifluoroethane and/or trichlorofluoroethane.
It is thus an object of the invention to provide a solely water blown integral skin polyurethane composition suitable for making shoe soles.
In particular, it is an object of the invention to provide an integral skin polyurethane composition capable of making integral skin polyurethane articles characterized by a tensile strength of greater than or equal to 450 psi and/or a Taber Abrasion (mg loss) of less than 200.
It is another object of the invention to provide integral skin polyurethane molded articles, suitable for making shoe soles, characterized by a tensile strength of greater than or equal to 450 psi and/or a Taber Abrasion (mg loss) of less than 200.
It is a further object of the invention to provide a process of making such integral skin polyurethane molded articles.
Finally, it is an object of the invention to provide an isocyanate composition for use in polyurethane compositions intended for use in the production of integral skin polyurethane articles suitable for use in shoe soles.
An advantage of the present invention is that a blowing agent consisting of water can be used in the production of integral skin polyurethane articles exhibiting performance properties which are at least equivalent to or better than those achieved with traditional blowing agents.